80 LESSONS m CHEMISTRY. 



waves. I do not include in the above list the sugar-cane," 

 banana, some other vegetables, fruit-trees, and imported 

 grasses. As these islands consist entirely of coral, and at 

 one time probably existed as a mere water-washed reef, all 

 the productions now living here must have been transported 

 by the waves of the sea. In accordance to this the flora has 

 quite the character of a refuge for the destitute : Professor 

 Henslow informs us, that of twenty species nineteen belong 

 to different genera, and these again to no less than sixteen 

 orders!" 



115. In the character and appearance of the soils and sub- 

 soils of every country there are certain striking differences 

 which immediately attract attention. What we have just 

 stated of the mode in which the layers of earth that compose 

 our fields have been produced will at once account for this 

 dissimilarity. It is evident that when the soil has been formed 

 by the crumbling down of a granite rock, it must differ con- 

 siderably in character from that which has been produced by 

 the decay of slate or limestone. When we take a quantity of 

 the soil of a field and expose it to heat in a spoon or any con- 

 venient vessel, we find that, like a plant, it blackens in colour, 

 that as we continue the heat the black portion disappears, 

 and there is left a quantity of fixed incombustible matter. 

 The weight of this incombustible part differs in different 

 soils, in some amounting to 98 per cent of the whole weight, 

 in others forming a considerably smaller proportion. The 

 part that burns and vanishes, usually termed the organic 

 portion, consists of the four elementary substances — carbon, 

 oxygen, hydrogen, and nitrogen — derived either from the 

 vegetables and animals that have lived and died upon the 

 soil, or that have been added to it in manure by the farmer, 

 while the incombustible matter is composed of the mineral 

 substances of the rocks, by the decay of which it had been 

 produced. This mineral portion, no matter from what kind 

 of rock derived, is invariably found to consist chiefly of three 

 substances. Silica, Lime, and a peculiar earth which we have 

 not yet described, termed Alumina. 



116. Alumina. Like lime and magnesia, alumina is a 

 compound of oxygen with a metal. It is a chief ingredient 

 of alum, and also of slate and pipe-clay, and exists pm'e in the 

 sapphire, ruby, and some other rare minerals. It may be pro- 

 cured by adding to a solution of the common alum of the 



